The permanent alteration of the color of keratinous fibers, in particular human hair, by the application of hair dyes is well known. Oxidative hair dye precursors, i.e., developers (also called primary intermediates) and couplers diffuse into the hair through the cuticle and into the cortex. The precursors combine in the presence of an oxidizing agent, such as hydrogen peroxide, to form larger-sized dye molecules that result in a visual color of the hair (“end hair color”) that is distinct from the color of the hair before coming into contact with the precursors and oxidizing agent. Different combinations of developers and couplers produce different shades of hair color when combined with the oxidizing agent. The end hair color is not easily predicted given the complex chemical process that occurs to achieve an end hair color.
Permanent hair colorant formulations should produce end hair colors that are stable for at least 4 to 6 weeks. Additionally, the end hair color should exhibit good washfastness, good lightfastness, fastness to rubbing, as well as sufficient resistance with respect to perspiration. Preferably, it should be possible to produce a broad palette of different color shades by combining suitable developers and couplers. Hair colorant formulations also should be stable, for example, in terms of shelf life, ability to retain color benefits after storage (i.e., stability of the developers and couplers), and chemical stability both after mixing and during application to hair.
Hair colorant formulations generally provide acceptable and immediate results. However, achieving both desired shade (hue) and depth of shade (chroma) remains challenging, particularly for hair colorant formulations that impart reddish colors to the hair. To achieve desirable shades in the red area, 5-amino-2-methylphenol (AHT):
is one common example of a suitable coupler, to be used by itself or in a mixture with other couplers, in combination with one or more suitable developer or developers. For example, when AHT is used as a coupler in combination with 4-aminophenol and a standard oxidant system such as ammonium hydroxide and hydrogen peroxide, red shades may be achieved. Nevertheless, the red shades may fail to provide the desirable red hue and chroma and, thereby, the hair colorant formulation may fall short of producing the desired end hair color.
GB Pat. No. 1,229,517 discusses a class of couplers having formula (A):

in which R of formula (A) is hydrogen, lower alkyl or hydroxyalkyl, acyl, of an N-alkylated or unsubstituted amino lower alkyl or carbamylmethyl group. The couplers are exemplified in combination with p-toluenediamine (2-methyl-1,4-benzenediamine) to produce colors such as violet, blue-grey, eggplant, violet-blue, reddish blond, and bluish black on white hair.
U.S. Pat. Appl. Pub. No. 2007/0067926 discusses agents for oxidative coloring of keratin fibers, in which 2-amino-5-ethylphenol is combined with at least one m-aminophenol. Exemplified compositions each comprise the 2-amino-5-ethylphenol and the m-aminophenol as couplers, in combination with one or more developers.
There remains an ongoing need for additional hair colorant compositions that can provide desirable red shades, particularly when used with other oxidant systems, i.e., ammonium carbonate, hydrogen peroxide, and, optionally, a radical scavenger and/or chelant. There remain further needs for hair colorant compositions that deliver not only redder shades with increased chromaticity, but also increased stability and shelf life.